The foregoing sludge usually contains a lot of water (approximately 99%), so that it is very difficult to compost it when it is raw. The sludge is banked up, is dehydrated until its moisture content becomes approximately 80%, and is changed to dehydrated pellets.
An existing solid-liquid separating system includes a cylindrical filter of a perforated metal and screws provided in the filter, and is generally called a screw press type dehydrator.
Sludge to be treated is put into the filter, is axially conveyed therein, and is discharged via holes of the perforated metal. In short, the sludge is extensively dehydrated as it being conveyed to a downstream side of the filter. Further, the filter is constructed to scrape the dehydrated sludge off from its inner surface, thereby getting ready to receive new sludge.
In the screw press type dehydrator, holes of the perforated metal tend to clog, and have to be cleaned by applying high-pressure water, which is troublesome and takes a high running cost.
It is very difficult to effectively use the whole circumferential area of the cylindrical filter. Immediately after the sludge is applied to the filter, approximately only a half of a circumferential area of the filter is usable.
In order to overcome the foregoing problems, parallel plate conveying type separating systems have been proposed in Japanese Patent No. 3,894,366 and Japanese Patent Laid-Open Publication Hei 6-155090.
With one of the parallel plate conveying type separating systems, a plurality of strip-shaped plates are piled and arranged in a direction which is orthogonal to a conveying direction of the sludge. A pair of combined plate groups interlock with each other. One of such combined plate groups performs a parallel circular movement, i.e. is displaced side to side and up and down, and conveys the sludge in a longitudinal direction in response to the displacement of the upper surface of the plates.
Each pair of upper and lower plate groups performs the parallel movement while their upper surfaces are displaced in the longitudinal direction, which enables the sludge to be gradually moved and conveyed on the plates.
Usually, gaps between each pair of interlocked plate groups are equally sized between an inlet and an outlet for the sludge. Treated liquid is discharged via the gaps.
With the foregoing parallel plate type separating system, the sludge is spread over substantially parallel upper flat surface of the plates. This is effective in improving a usage rate of the filter surface. Further, the plates rub with each other during the parallel movement, which can prevent clogging of the filter, and is substantially free from maintenance work such as cleaning.